Photosynthesis uses the energy of the sun with water as the reducing agent to drive the reduction of carbon dioxide to carbohydrates with oxygen as a co-product through a remarkably complex process. At photosystem II (PSII), a subsystem imbedded in the thylakoid membrane where O2 is produced, light absorption, energy migration, electron transfer, proton transfer, and catalysis are all utilized in multiple stepwise chemical reactions which are carefully orchestrated at the molecular level.
Photosynthesis solves the problem of energy storage by biomass production but with low solar efficiencies, typically <1%. In artificial photosynthesis with solar fuels production, the goal is similar but the targets are either hydrogen production from water splitting, reaction 1, or reduction of carbon dioxide to a carbon-based fuel, reaction 2. Different strategies for solar fuels have evolved. In one, direct bandgap excitation of semiconductors creates electron-hole pairs which are then used to drive separate half reactions for water oxidation (2H2O→O2+4H++4e−) and water/proton reduction (2H++2e−→H2).2H2O+4hv→O2+2H2  (1)2H2O+CO2+6hv→CH3OH+3/2O2  (2)